At the time of writing this, a total of 12 missions are active, split between all the difficulties 1 to 5. Add modifiers and side objectives on top, and you can easily end up with a situation where there are only missions available for a given difficulty that you would rather not play right now, be that for personal preferences or other reasons.
In particular, I think for each difficulty there should always be one vanilla mission open without any modifiers or side objectives at all.
(For example, for difficulty 2, there is right now one mission with the endless horde modifier, and another with the grimoires side objective. So you either get a mission that is just harder for no benefit, or a mission that is harder and extremely obnoxious for very little benefit, but not a single regular unmodified mission.)